1. Field of the Invention
This invention is in the field of manufacturing transparent, conductive indium-tin oxide layers used as electrodes for image sensor arrays by means of low pressure deposition on to an amorphous, hydrogen-containing silicon layer and subsequent heat treatment in an oxygen atmosphere.
2. Decription of the Prior Art
It is known that a cover electrode for image sensor arrays must exhibit a high transparency on the order of at least 85% for visible light and must simultaneously exhibit a low electrical sheet resistance of less than 300 ohms/square. Such specifications can only be met by indium-tin oxide (ITO) layers that have either been deposited at temperatures above 200.degree. C. or have been heat treated at temperatures above 440.degree. C. (see the report by Hamberg, et al., J. Appl. Phys. 60 (11) 1986, R 123 thru R 129).
For heating of the indium-tin oxide layer in the manufacture of image transducers, the following must be taken into consideration during the layer deposition or during the subsequent heat treatment:
1. In order to be able to structure the ITO layer photolithographically after deposition, the deposited material must not be exposed to a temperature higher than about 150.degree. C. A higher temperature treatment would reduce the chemical solubility of the ITO such that a residue-free etching of the structure is no longer possible. In the alternative method for structuring, what is referred to as a lift-off technique, the substrate must be covered with a structured photoresist during the coating and can no longer be further worked after exceeding a temperature of about 100.degree. C., i.e., it can no longer be lifted off by organic solvents.
2. Above 300.degree. C., the photoconductive, amorphous, hydrogen-containing silicon underneath the ITO layer loses its hydrogen content which is critical for its semiconducting properties. The heat treatment at 440.degree. C. that could lead to meeting the required specifications in the case of cold-deposited and completely structured ITO layers can therefore not be applied (see the report by Raviendrad, et al., Physica Status Solidi (a) 88 (1985) K83 through K86, and the previously cited report by Hamberg, et al.).
In addition to the noted physical restrictions, a heating and subsequent cooling of substrates in the course of manufacturing the same represents a considerable complication in view of the costs and the reduction in throughput times.
When the standard ITO manufacturing methods are modified to meet the restrictions, the specifications of the ITO are rendered noticeably poorer. The performance capability of the image sensors is thereby noticeably decreased.
A method which is likewise concerned with this problem is set forth, for example, in European Patent Application No. 0 217 095. In order to improve the values of resistance (300 ohms per square) with unaltered transparency (90%) the heat treating in this method is performed in two steps at a maximum of 200.degree. C., the first heat treating being carried out in an oxygen atmosphere for achieving the transparency and secondly in a plasma-activated forming gas atmosphere for determining the sheet resistivity.